Method of detecting a change in the property of a rubber member

ABSTRACT

The electrical impedance of a dumbbell-shaped rubber sheet (1) is measured, and the deterioration of the dumbbell-shaped rubber sheet (1) is detected from the result of the measurement. The present invention relates to a method of detecting a change in the property of a rubber member or a rubber member of a rubber bearing, such as a change in the shape or dimensions thereof, a change in the state of the stress and strain thereof, deterioration thereof, and the like.

This is a continuation of application Ser. No. 08/379,591, filed Feb. 1,1995, now abandoned.

TECHNICAL FIELD

The present invention relates to a method of detecting a change in theproperty of a rubber member or a rubber member of a rubber bearing, suchas a change in the shape or dimensions thereof, a change in the state ofstress and strain thereof, deterioration thereof, and the like.

BACKGROUND ART

A rubber bearing which is comprised of a rubber member and a reinforcingmember is used in supporting a structure such as a building and thelike. As a result of the fact that such rubber bearings are generallydisposed in a lower portion of the structure, the rubber bearingsreceive the load of the structure over a long period of time, and arealso subjected to shear stress due to earthquakes and the like, whilethey gradually deteriorate due to aging or the like. When thedeterioration of a fixed level or more has occurred, it is necessary toreplace the rubber bearings, and the extent of the deterioration of therubber member of the rubber bearing is conventionally ascertained byvisual observation of a configuration or dimensional change in theoverall configuration of the rubber member, a crack or an exfoliation inthe rubber member or the like, or by the use of a measuring instrumentsuch as a strain gauge, an ultrasonic test instrument, or a measure.

Since the rubber bearings are generally disposed in a lower portion ofthe structure as described above, the visual observation of the rubbermembers is very difficult since an inspector must enter the lowerportion of the structure and conduct the visual observation. Inaddition, the inspector must rely on his experience and intuition in theobservation, so that there is a possibility that the results ofobservation may vary. Meanwhile, in the observation of the rubbermembers by the use of measuring instruments such as those describedabove, it is possible to obtain satisfactory results to a certain extentwhen a dimensional change is involved. When a dimensional change is notinvolved, it is impossible to obtain desired results. In order tomonitor the supporting performance of the rubber bearings during theperiod of use, it is necessary to remove the rubber bearings from thestructure, or separately prepare in advance a rubber bearing specimen ofthe same material as that of the rubber bearings installed under thestructure, and to conduct a dynamic test by applying a fixed axial forceto the rubber bearing specimen after the lapse of a predeterminedperiod. Thus, it has been impossible to monitor the supportingperformance of the rubber bearings in use as installed under thestructure.

Such a problem occurs not only with the rubber member itself of therubber bearing, but, generally speaking, it can similarly occur withrubber members applied to various members which are formed of materialsuch as natural rubber, synthetic rubber, or the like.

The present invention has been devised in view of the above-describedaspects, and its object is to provide a method of detecting a change inthe property of a rubber member, the method being capable of reliablyobserving the extent of deterioration even when a dimensional change isnot involved, and in the case of a rubber member of a rubber bearing,making it unnecessary for an inspector to enter a lower portion of astructure at the time of observation, and making it possible to reliablyascertain the extent of deterioration remotely.

DISCLOSURE OF INVENTION

In accordance with the present invention, the above-described object isattained by a method comprising the steps of: measuring an electricalimpedance of a rubber member; and detecting a change in a property ofthe rubber member from a result of the measurement.

In addition, the above object is also attained by a method comprisingthe steps of: measuring an electrical impedance of a rubber member of arubber bearing which comprises the rubber member and a reinforcingmember; and detecting a change in a property of the rubber member of therubber bearing from a result of the measurement.

Here, in a case where the reinforcing member is made of a metal, theelectrical impedance of the rubber member may be measured by using thereinforcing member as an electrode. Furthermore, in a case where therubber bearing further has flange plates respectively disposed onopposite end surfaces of the rubber member, the electrical impedance ofthe rubber member may be measured by using the flange plates aselectrodes.

The changes of the properties which can be detected by the presentinvention include a dimensional change, a change in the state of stressand strain, deterioration, and the like, and it is possible to cite thedeterioration, in particular.

The material of the rubber member, which is the object to be measured inthe present invention, may be natural rubber, synthetic rubber, or amixture thereof. In the case of the rubber bearing which is used in anearthquake-resistant device, an isolator, and a vibration-controldevice, there are cases where high-damping rubber is used, and themethod of the present invention can be suitably applied to the rubbermember made of such a high-damping rubber. The rubber bearing used inthe earthquake-resistant device, the isolator, and the vibration-controldevice is formed with a lead plug additionally disposed in the center ofthe rubber member, as necessary, and the method of the present inventioncan be suitably applied to such a rubber bearing incorporating the leadplug.

In the present invention, electrodes are disposed such that the rubbermember whose change in the property is to be detected is placedtherebetween. A voltage of, for instance, 200 kHz is applied acrossthese electrodes, and the electrical impedance of the rubber member ismeasured from a current value. Here, the electrical impedance changes incorrespondence with the change in the property of the rubber member,e.g., the generation of the internal strain, so that the change in theproperty of the rubber member can be detected from the result of themeasurement.

As described above, in accordance with the present invention, even whena dimensional change is not involved, it is possible to reliably observethe extent of deterioration of the rubber member, and, in the case of arubber bearing, it is unnecessary for an inspector to enter a lowerportion of the structure at the time of observation, and it is possibleto reliably ascertain the extent of deterioration remotely.

Hereafter, a description will be given of the present invention withreference to the embodiments shown in the drawings. It should be notedthat the present invention is not limited to these embodiments.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a plan view of a rubber sheet used in Example 1 for explainingthe method of the present invention;

FIG. 2 is a front elevational view explaining a measuring method inaccordance with Example 1;

FIG. 3 is the result of measurement of the relationship between thedisplacement of the rubber sheet and the impedance between electrodes inExample 1;

FIG. 4 is the result of measurement of the relationship between a forceapplied to the rubber sheet and the impedance between the electrodes inExample 1;

FIG. 5 is the result of measurement of the relationship between thedisplacement of the rubber sheet and the force applied to the rubbersheet in Example 1;

FIG. 6 is the result of measurement of the relationship between theelapsed time and the impedance between the electrodes in Example 1;

FIG. 7 is the result of measurement of the relationship between theelongation and the thickness in Example 1;

FIG. 8 is a cross-sectional view of a rubber bearing used in Example 2for explaining the method of the present invention;

FIG. 9 is the result of measurement of the relationship between avertical displacement occurring to the rubber member and a verticalforce applied to the rubber member in Example 2;

FIG. 10 is the result of measurement of the relationship between thevertical displacement occurring to the rubber member and the impedancebetween inner steel plates in Example 2;

FIG. 11 is the result of measurement of the relationship between avertical force applied to the rubber member and the impedance betweenthe inner steel plates in Example 2;

FIG. 12 is the result of measurement of the relationship between theelapsed time and the impedance between the electrodes in Example 2; and

FIG. 13 is a cross-sectional view of another example of a rubber bearingto which the method of the present invention is applicable.

EMBODIMENTS EXAMPLE 1

A dumbbell-shaped rubber sheet 1 of No. 1 type of JIS (K6301) such asthe one shown in FIG. 1 was prepared as an object to be measured. As forthis rubber sheet 1 as a rubber member, its overall length L was 120 mm,the length l of its small-width portion was 40 mm, the width W of itslarge-width portion was 25 mm, the width w of its small-width portionwas 10 mm, and its thickness t was 2 mm. Then, as shown in FIG. 2,electrodes 2 and 3, which were formed of copper plates having a diameterof 5 mm and a thickness of 1 mm, were applied to both sides of therubber sheet 1, and they were clamped and held by polyethylene plates 4and 5. Under the conditions of the temperature: 25° C., a deformationrate: 10 mm/min, and a pressing force: 3 windings, an elongating forcein the direction of arrow A (in the longitudinal direction) was appliedto this rubber sheet 1 repeatedly four times such that the elongationpercentage (100%=50 mm) became 0%-25%-0%-50%-0%-75%-0%-100%-0%. At thesame time, a voltage with a frequency of 200 kHz was applied across theelectrodes 2 and 3. The results of measurement are shown in FIGS. 3 to6.

Here, FIG. 3 shows the result of measurement of the relationship betweenthe displacement of the rubber sheet 1 and the impedance between theelectrodes 2 and 3. FIG. 4 shows the result of measurement of therelationship between the force applied to the rubber sheet 1 and theimpedance between the electrodes 2 and 3. FIG. 5 shows the result ofmeasurement of the relationship between the displacement of the rubbersheet 1 and the force applied to the rubber sheet 1. FIG. 6. shows theresult of measurement of the relationship between the elapsed time andthe impedance between the electrodes 2 and 3. Incidentally, as themeasuring instrument, the LF Impedance Analyzer (Model 4129Amanufactured by Yokogawa-Hewlett-Packard, Ltd.) was used.

Focusing attention on FIG. 6, it can be seen that the impedance changesin the order of a, b, c, d, e, f, g, h, and i in correspondence with theapplication of the elongating force of 0%-25%-0%-50%-0%-75%-0%-100%-0%.Also, it can be seen that the impedances c, e, g, and i are respectivelygreater than the original impedance a, and that the impedance graduallyincreases in the order of c, e, g, and i in correspondence with anincrease in the number of times the elongating force is applied.Accordingly, by detecting the change in impedance from the originalimpedance, it is possible to detect the application of the elongatingforce to the rubber sheet 1, i.e., the occurrence of the internal strainin the rubber sheet 1. In addition, by detecting the amount of increasein impedance, it is possible to detect the extent of deterioration ofthe rubber sheet 1 ascribable to the generation of the repeated internalstrains. The large changes exhibited by the impedances b, d, f, and hare conceivably attributable to the changes in the thickness of therubber sheet 1 due to the elongation. For reference, FIG. 7 shows theresult of measurement of the relationship between elongation andthickness.

EXAMPLE 2

A rubber bearing 10 such as the one shown in FIG. 8 was prepared. Therubber bearing 10 is comprised of disk-shaped upper and lower connectingsteel plates (upper and lower flange plates) 11 and 12, a cylindricalrubber member 13, and three disk-shaped inner steel plates 14, 15, and16 serving as reinforcing members embedded in the rubber member 13. Leadwires are respectively screwed to the inner steel plates 14 and 15 ofthe rubber bearing 10, the lead wires were led to the outside, and theinner steel plates 14 and 15 were used as electrodes. Under thetemperature condition of 23° C., a uniform stationary compressive forcewith a surface pressure of 40 kgf/cm² and a load of 4.46 tonf wasapplied to the rubber bearing 10 repeatedly three times in the directionof arrow B (in the vertical direction). At the same time, a voltage witha frequency of 100 kHz was applied across the inner steel plates 14 and15. The results of measurement are shown in FIGS. 9 to 12.

FIG. 9 shows the result of measurement of the relationship between thevertical displacement occurring in the rubber member 13 and the verticalforce applied to the rubber member 13. FIG. 10 shows the result ofmeasurement of the relationship between the vertical displacementoccurring in the rubber member 13 and the impedance between the innersteel plates 14 and 15. FIG. 11 shows the result of measurement of therelationship between the vertical force applied to the rubber member 13and the impedance between the inner steel plates 14 and 15. FIG. 12shows the result of measurement of the relationship between the elapsedtime and the impedance between the inner steel plates 14 and 15.Incidentally, a measuring instrument similar to the one used in Example1 was used as the measuring instrument.

Focusing attention on FIG. 12, it can be seen that the impedance changesin the order of a, b, c, d, e, f, and g in correspondence with thecompressive load applied repeatedly three times in the order of 0-4.46tonf-0-4.46 tonf-0-4.46 tonf-0, and that the change in impedance issmall when the compressive load is reduced. Accordingly, by detectingthe change in impedance, it is possible to detect the application of avertical force to the rubber member 13, i.e., the generation of theinternal strain in the rubber member 13. In addition, by performing theintegration and the like of the times the impedance changes, it ispossible to detect the extent of the deterioration of the rubber member13 ascribable to the generation of the repeated internal strain. As aresult, it is possible to readily ascertain the extent of thedeterioration of the rubber member 13 by introducing the lead wires to aremote place, e.g., a monitoring chamber or the like, and by monitoringthe change in impedance there.

It should be noted that the method of the present invention is alsoapplicable to, in addition to the rubber bearing 10 such as the onedescribed above, a rubber bearing 53 comprised of a rubber member 51 andan inner steel plate 52 serving as a reinforcing member embedded in therubber member 51, as shown in FIG. 13, by, for example, attachingelectrodes to one end surface 54 of the rubber member 51 and the otherend surface 55 opposed thereto, respectively.

We claim:
 1. A method for detecting a deterioration of a rubber bearingwhich comprises a load supporting rubber member, a metal reinforcingmember disposed in the rubber member, and flange plates respectivelydisposed on opposite end surfaces of the rubber member, wherein saidmethod comprises the steps of:disposing said rubber bearing in a lowerportion of a building to receive a load from the building; repeatedlymeasuring electrical impedance of the load supporting rubber memberunder normal stress states due to normal load from said building afterexternal stresses have been repeatedly applied to the rubber bearing dueto a change of said load from said building, to provide electricalimpedance measurements of the rubber member under the normal stressstates, respectively; comparing a preceding electrical impedancemeasurement of the rubber member with a subsequent electricalmeasurement of the rubber member subsequent to said preceding electricalimpedance measurement, the preceding electrical impedance measurementbeing a value under the normal stress state of the rubber bearing aftera preceding external stress has been applied to the rubber bearing, thesubsequent electrical impedance measurement being a value under thenormal stress state of the rubber bearing after a subsequent externalstress to said preceding external stress has been applied to the rubberbearing; detecting changes of the subsequent electrical impedancemeasurement from the preceding electrical impedance measurement from aresult of the comparison; and determining a deterioration of the rubbermember based on said external stresses applied to the rubber bearingfrom a result of the detection.
 2. A method according to claim 1,wherein the electrical impedance is measured through said reinforcingmember as an electrode.
 3. A method according to claim 1, wherein theelectrical impedance is measured through said flange plates aselectrodes.
 4. A method according to claim 1, further comprising a stepof integrating times of change of the electrical impedance measurement,said determining step further including determining deterioration of therubber member based on said external stresses applied to the rubberbearing, in response to the integrated times of the electrical impedancemeasurements.
 5. A method for detecting a deterioration of a rubberbearing which comprises a load supporting rubber member, a metalreinforcing member disposed in the rubber member, and flange platesrespectively disposed on opposite end surfaces of the rubber member,wherein said method comprises the step of:disposing said rubber bearingin a lower portion of a building to receive a load from the building;repeatedly measuring electrical impedance of the load supporting rubbermember under normal stress states due to normal load from said buildingbefore an external stress is applied to the rubber bearing and afterexternal stresses have been repeatedly applied to the rubber bearing,due to a change of said load from said building, to provide electricalimpedance measurements of the rubber member under the normal stressstates, respectively; repeatedly comparing an original electricalimpedance measurement of the rubber member with subsequent electricalmeasurements of the rubber member subsequent to said original electricalimpedance measurement, the original electrical impedance measurementbeing a value under the normal stress state of the rubber bearing beforea first external stress is applied to the rubber bearing, the subsequentelectrical impedance measurements being values under the normal stressstate of the rubber bearing after the external stresses have beenapplied to the rubber bearing, respectively; detecting changes of eachof the subsequent electrical impedance measurements from the originalelectrical impedance from a result of the comparison; and determining adeterioration of the rubber member based on said external stressesapplied to the rubber bearing from a result of the detection.
 6. Incombination, a rubber bearing having a load supporting rubber member, ametal reinforcing member disposed in the rubber member, and flangeplates respectively disposed on opposite end surface of the rubbermember, and an apparatus for detecting a deterioration of the rubberbearing, said rubber bearing being disposed in lower portion of abuilding to receive a load from the building, the apparatusincluding:measuring means for repeatedly measuring electrical impedanceof the load supporting rubber member under normal stress states due tonormal load from said building before an external stress is applied tothe rubber bearing and after external stresses have been repeatedlyapplied to the rubber bearing, due to a change of said load from saidbuilding, to provide electrical impedance measurements of the rubbermember under the normal stress states, respectively; comparing means forrepeatedly comparing an original electrical impedance measurement of therubber member from said measuring means with subsequent electricalmeasurements of the rubber member from said measuring means subsequentto said original electrical impedance measurement, the originalelectrical impedance measurement being a value under the normal stressstate of the rubber bearing before a first external stress is applied tothe rubber bearing, the subsequent electrical impedance measurementsbeing values under the normal stress state of the rubber bearing afterthe external stresses have been applied to the rubber bearing,respectively; detecting means for detecting changes of each of thesubsequent electrical impedance measurements form the originalelectrical impedance from a result the comparison of said comparingmeans; and determining means for determining a deterioration of therubber member based on said external stresses applied to the rubberbearing from a result of the detection of said detecting means.